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Consider the following order of acidity based on pKa values:-

ethanoic acid > phthalimide >phenol >water >acetamide

Looking towards similar questions on stack exchange such as this , the recurring theme is that both quality(the electronegativities of the atoms on which charge is delocalized)as well as quantity of resonance(the number of resonating structures) should be seen when determining the stability of conjugate base and quality is given precedence over quantity. However, this doesn't seem to work here. Let me categorize the conjugate bases of the aforementioned compounds:-

  1. Ethanoic Acid- Delocalisation over 2 oxygen atoms
  2. Phtalimide- Delocalisation over 2 oxygen atoms and 1 nitrogen atom
  3. Phenol-Delocalisation over 1 oxygen atom and 3 carbon atoms
  4. Water-Localised over 1 oxygen atom
  5. Acetamide- Delocalisation over 1 oxygen atom and 1 nitrogen atom

Now, based on the linked question, we get ethanoic acid > phenol > water where we have seen quality of resonance first then quantity. However, this doesn't work for phtalimide which should be placed above carboxylic acid and also acetamide which should be placed above water and possibly even phenol.

So how would you explain this order?

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    I think you have this whole thing backwards. From the experimental acidity, we give these rule-of-thumb explanations, which fit to varying degrees. You shouldn't expect the other way around to work in general. New chemistry students get too caught up in these rules and overestimate their explanatory power and precision. – Nicolau Saker Neto Jun 11 '22 at 06:42
  • @NicolauSakerNeto Agreed, but there must be some explanation given for experimental results, right? Is the explanation in this case too advanced(like based on the schridinger equation) or can it be explained by basic principles? –  Jun 11 '22 at 06:53
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    Indeed. The accurate computation of substance acidities purely from first principles, especially outside of the gas phase, is still a matter of active research. – Nicolau Saker Neto Jun 11 '22 at 07:24
  • https://chemistry.stackexchange.com/questions/94870/why-is-ethanoic-acid-more-acidic-than-phthalimide?rq=1 – Mithoron Jun 11 '22 at 12:03
  • (...) quality is given precedence over quantity. However, this doesn't seem to work - doesn't it? See you're comparing apples and oranges here. N acid is not O acid. If imidic acids were major tautomers and not amides then your logic could been OK. – Mithoron Jun 11 '22 at 12:21
  • BTW this is pretty much your earlier q. all over again https://chemistry.stackexchange.com/questions/165706/why-are-amides-weaker-acids-than-water – Mithoron Jun 11 '22 at 12:25
  • @Mithoron so is the atom to which H is directly connected to of greater importance than all the atoms upon which the charge resonates? –  Jun 11 '22 at 12:32
  • More precisely it depends which mesomeric structure structure is the major contributor. You were assuming there were equal, while apparently the one with charged nitrogen is more important. – Mithoron Jun 11 '22 at 12:38
  • @Mithoron Why is the one with charged nitrogen more important when the charge is more stable on oxygen? –  Jun 11 '22 at 12:40
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    I assume it the matter of stronger C=O then C=N – Mithoron Jun 11 '22 at 12:41

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